The most common Zerg openings are 14 pool and 15 pool. 17 nexus 17 forge is safe against those openings on most of the ladder maps except for close spawns. Against a 14 pool, the lings can show up just a few seconds before your cannon finishes, so you may need to complete your wall-off with a pylon and then cancel when your cannon finishes.

17 nexus is significantly more economical than 14 forge. If you open 14 forge, you need to pylon block Zerg's natural and probe block his third or you'll be behind economically.

Against earlier pools, you need the forge first. You can handle this by either scouting after pylon or just crossing your fingers. At my level on the ladder, I see 14 and 15 pools about 90% of the time, but it might be different in the lower levels where players like 6 pools and 10 pools more.

Wow, this is definitely my new PvZ build. So far I've tried it probably five times, and the only time I lost was my first attempt. Granted, I'm in gold, so my execution is pretty shit, but it works well regardless.